1. Field of the Invention
The present invention relates to image forming apparatuses such as copying machines, facsimile machines, and printers.
2. Description of the Related Art
Generally, an image forming apparatus using an electro photographic process includes an image forming station in which a latent image formed using light, magnetism, electric charges, or the like is rendered visible and thereby a visible image is obtained. The visible image is transferred to a recording sheet fed to the image forming station. FIG. 7 shows an example of the color image forming apparatus, which is structured such that four image forming stations Pa, Pb, Pc, and Pd include photosensitive drums 1a, 1b, 1c, and 1d, respectively serving as image bearing members. Disposed around the photosensitive drums 1a, 1b, 1c, and 1d in the direction of the drum rotation are: exposure portions 2a, 2b, 2c, and 2d; developing portions 3a, 3b, 3c, and 3d; primary transfer portions 4a, 4b, 4c, and 4d, each of which includes a drum and roller in combination; and cleaning portions 5a, 5b, 5c, and 5d. Below the photosensitive drums 1a to 1d, an intermediate transfer belt 6 is stretched round a series of rollers in an endless flat belt. A roller 7, which is one of the rollers, is used as a drive motor, to which rotating power is input from a drive source motor (not shown). Disposed around the intermediate transfer belt 6 in the running direction of the belt, as shown by the arrow in the drawing, are a registration detection sensor 8, a secondary transfer portion including a pair of rollers 9 and 11, an intermediate transfer cleaning portion 10, and the like.
With this configuration, a color image is formed as follows. First, in the exposure portion 2a of the first image forming station Pa, a latent image for a yellow (Y) component color of the image of an original is formed on the photosensitive drum 1a by an electrophotographic means. Subsequently, in the developing portion 3a, the latent image is rendered visible by developer that has Y color toner. In the primary transfer portion 4a, the Y color toner image formed by rendering the latent image visible is transferred onto the intermediate transfer belt 6. In the course of transfer of the Y color toner image to the intermediate transfer belt 6, in the second image forming station Pb, a latent image for a magenta (M) component color is formed on the photosensitive drum 1b in the same manner and, subsequently, a magenta toner image is formed with M color toner in the developing portion 3b. When the intermediate transfer belt 6 to which the Y color toner image is transferred in the first image forming station Pa reaches the primary transfer portion 4b in the second image forming station Pb, an M color toner image is transferred to the predetermined part on the intermediate transfer belt 6 to which the Y color toner image has been transferred.
An image for a cyan (C) color and an image for a black (Bk) color are also formed in the same manner. After the four toner images of the four different colors are superposed one on another on the intermediate transfer belt 6, the intermediate transfer belt 6 is further moved so that the toner images reach the secondary transfer portion 9, 11, where the toner images are transferred all at once onto a sheet fed to the secondary transfer portion. The sheet on which the toner images formed by the above-described image forming means have been transferred all at once is fed to a conveying mechanism 20. In the conveying mechanism 20, the sheet is placed onto a conveying belt, through which a large number of suction holes are made; and the sheet is fed to a fixing device 13 while sheet is drawn and held on the belt by a suction fan. In the fixing device 13, the toner images are fixed onto the sheet under pressure and heat applied by a fixing roller pair and an internal heater. After such a fixing process, the sheet is ejected from the main body of the apparatus.
In the field of the color image forming apparatuses, such as that shown in FIG. 7, demands for higher image quality and higher speed image output have been growing exponentially recently.
In order to produce an image having a high image quality at a high speed, it is necessary to enhance the capability of fixing toner to a recording sheet. A structure in which two or more fixing units is disposed on a sheet conveyance path and the sheet is passed through fixing nips in sequence to improve fixing performance is required (Japanese Patent Application Laid-Open No. 6-258970). It may be necessary that the size of a fixing unit be increased to enhance fixing capability.
Such an increase in the size of a fixing means in order to improve the fixing capability may greatly affect transport of an image forming apparatus structured such that the image forming means and the fixing means are accommodated in one accommodating body. For example, the image forming apparatus cannot be carried into an elevator. In view of such a case, a structure has been proposed that allows the separation of an image forming device including image forming stations and the like, and a fixing device including a fixing unit and the like (Japanese Patent Laid-open No. 2001-183959).
Even in such a configuration, heat is conducted from the fixing unit accommodating body to the image forming accommodating body. In order to prevent such heat conductor, the configuration in which an air duct is provided between the fixing unit and the image forming unit is conventionally known (JP-A No. 2003-280414).
However, in the case where the fixing accommodating body and the image forming accommodating body are separated from each other, the provision of such an air duct in either the fixing accommodating body or image forming accommodating body leads to another problem, that is an increase in the number of components.